A Sprite on a TileA TiledSprite represents a sprite's position using tile coordinates (xTile, yTile); its most important method allows a sprite to move from its current tile to an adjacent one using a compass direction (quadrant): NE, SE, SW, NW. One assumption of TiledSprite is that a sprite cannot move around inside a tilethe sprite can only step from one tile to another. The constructor initializes a sprite's tile position after checking its validity with WorldDisplay: protected int xTile, yTile; // tile coordinate for the sprite protected WorldDisplay world; public TiledSprite(int x, int y, int w, int h, ImagesLoader imsLd, String name, WorldDisplay wd) { super(0, 0, w, h, imsLd, name); setStep(0, 0); // no movement world = wd; if (!world.validTileLoc(x, y)) { // is tile (x,y) valid System.out.println("Alien tile location (" + x + "," + y + ") not valid; using (0,0)"); x = 0; y = 0; } xTile = x; yTile = y; } // end of TiledSprite( ) Moving to Another TileAlienTiles' staggered tile layout means that the coordinates of the four tiles adjacent to the current one are obtained in different ways, depending on if the current tile is on an even or odd row. Figures 13-12 and 13-13 show examples of the two possibilities. The highlighted tile in Figure 13-12 is in row 3 (odd), and the one in Figure 13-13 is in row 2 (even). The coordinates of the adjacent tiles are calculated differently in these two cases. Figure 13-12. Moving from tile (1,3)Figure 13-13. Moving from tile (1,2)tryMove( ) calculates a new tile coordinate based on the current location and the supplied quadrant. A four-way branch deals with the four possible directions, and each branch considers whether the starting point is on an even or odd row: public Point tryMove(int quad) { Point nextPt; if (quad == NE) nextPt = (yTile%2 == 0)? new Point(xTile,yTile-1) : new Point(xTile+1,yTile-1); else if (quad == SE) nextPt = (yTile%2 == 0)? new Point(xTile,yTile+1) : new Point(xTile+1,yTile+1); else if (quad == SW) nextPt = (yTile%2 == 0)? new Point(xTile-1,yTile+1) : new Point(xTile,yTile+1); else if (quad == NW) nextPt = (yTile%2 == 0)? new Point(xTile-1,yTile-1) : new Point(xTile,yTile-1); else return null; if (world.validTileLoc(nextPt.x, nextPt.y)) // ask WorldDisplay if proposed tile is valid return nextPt; else return null; } // end of tryMove( ) The method is called tryMove( ) since there is a possibility that the desired quadrant direction is invalid because the new tile is a no-go area (it is occupied by a block) or the coordinate lies off the surface. These cases are checked by called validTileLoc( ) in WorldDisplay. |